1. Field of the Invention
This invention relates to display devices, especially ones that use an organic material for light emission.
2. Description of Related Technology
One type of electroluminescent display device is described in PCT/WO90/13148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(p-phenylenevinylene)xe2x80x94xe2x80x9cPPVxe2x80x9d) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. The electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
Another type of organic light-emitting device is a small molecule device, details of which are given in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference. These have a light-emitting layer which comprises at least one small molecule material such as tris(8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d) sandwiched between the two electrodes.
In an organic light-emitting display device the organic light-emitting layer is generally divided into individual pixels, which can be switched between emitting and non-emitting states by altering the current flow through them. The pixels are generally arranged in orthogonal rows and columns. Two alternative arrangements for controlling the pixels are generally used: passive matrix and active matrix. In a passive matrix device one of the electrodes is patterned in rows and the other in columns. Each pixel can be caused to emit light by applying an appropriate voltage between the row and column electrodes at whose intersection it lies. In an active matrix display circuitry is provided so that each pixel can be left in an emitting state whilst another pixel is addressed.
FIG. 1 shows a schematic cross-section through an active matrix organic light emitting device. The device is based on a glass sheet 1 which is covered with a passivation layer 2. Each pixel has a ion of circuitry 3, comprising thin-film transistors (TFTs), for regulating the supply of current to the pixel. The output of the circuitry 3 is provided to transparent anode electrode 4, which is spaced from the glass. Behind the anode 4 lies at least one layer 6 of light-emitting organic material. A cathode electrode 7 is provided behind the light-emitting layer 6. Banks 8 of insulating material are provided to separate neighbouring light-emitting regions and to insulate the rear of the circuitry 3. When the circuitry 3 is controlled to turn the pixel on, current is supplied to the anode 4 and flows through the light emitting layer to the cathode 7, causing light to be emitted.
TFT devices are well-known in the field of LCD displays. In that field work has been done in with the aim of improving contrast by reducing undesired reflection of ambient light from LCD displays. It has been suggested that black material should be located in line (in the viewing direction) with the TFT circuitry to protect the TFTs by absorbing incident light and also blocking light from any backlight fitted to the display. Examples of such proposals are disclosed in JP 57-18364, JP 61-116324, JP 4-225328, JP 5-107550, JP 5-173183, JP 6-301052 and JP 8-152612.
For similar reasons it has been proposed in JP application number 9-57862 to provide a black material behind a thin metal electrode (analogous to cathode 7 above). However, this does not protect the TFT circuitry from interference from incident light. Nor does it solve a specific problem of organic light-emitting devices, which is that, unlike e.g. typical LCD displays, the pixels of organic light-emitting devices normally emit light at a wide angular spread. Light that is emitted particularly widely can be waveguided by the glass cover sheet 1, as illustrated by arrow A in FIG. 1. This trapped light reduces the efficiency of the display and causes cross-talk between neighbouring pixels and increases the exposure of the TFTs to light from the pixels themselves.
According to a first aspect of the present invention there is provided an organic light-emitting device comprising: a transparent cover sheet; a region of organic light-emitting material behind the cover sheet; a region of circuitry behind the cover sheet for regulating the flow of current to the organic light-emitting material; and a non-light-transmissive layer which lies between the cover sheet and the circuitry.
The non-light transmissive layer may be light-absorbent and/or light-reflective.
The non-light-transmissive layer is preferably non-light-transmissive (e.g. light-absorbent) in (and preferably throughout) the visible frequency range, most preferably non-light-transmissive at the frequency at which the light-emitting material emits light. The non-light-transmissive layer is suitably a low light reflectance and/or high light absorption layer. The non-light-transmissive layer preferably has a reflectance of less than 30%, 20%, 15% or 10% at visible light wavelengths. The layer may, for example, be black or brown or another colour.
Preferably the non-light-transmissive layer is adjacent to the cover sheet, and most preferably the major surface of the non-light-transmissive layer lies in contact with the cover sheet. Preferably the non-light-transmissive layer lies between all of the region of circuitry (suitably including any or all data, signal etc. lines) and the cover sheet, to inhibit light from outside the device from reaching the circuitry and reducing contrast. Preferably none of the non-light-transmissive layer lies between the light-emitting region and the cover sheet, so as to allow light from the light-emitting region to leave the device. The non-light-transmissive layer may thus define a light-transmissive hole whose location corresponds to the region of light-emitting material. The non-light-transmissive region may thus frame the region of light-emitting material.
The device may comprise a plurality of regions of organic light-emitting material behind the cover sheet, which suitable correspond to pixels or sub-pixel units of the device. The device may comprise a plurality of regions of circuitry behind the cover sheet, each for regulating the flow of current to a respective one of the regions of organic light-emitting material. The or each organic light-emitting region is suitably framed by the non-light-transmissive layer; therefore, the non-light-transmissive layer may be of a lattice configuration, defining a regularly-spaced array of light-transmissive regions each corresponding to a respective one of the regions of organic light-emitting material.
The non-light-transmissive layer suitably comprises a metal, preferably a refractory metal. The non-light-transmissive layer may comprise an alloy. The non-light-transmissive layer may comprise a non-light-transmissive metal oxide, preferably a refractory metal oxide. The oxide is suitably a non-stoichiometric metal oxide. The non-light-transmissive layer may comprise a chromium oxide.
The or each light-emitting region suitably comprises a light-emitting polymer material, preferably a conjugated material. A suitable material is a semiconductive conjugated polymer such as PPV or a derivative thereof. The light-emitting material of which the or each light-emitting region is formed suitably is or comprises PPV, poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers. It could be deposited by spin-coating, dip-coating, blade-coating, meniscus-coating, self-assembly, ink-jet printing etc. The constituent of the light-emitting region and/or its precursor may be water-based: examples are precursor-based PPVs. Alternative materials include organic molecular light-emitting materials, e.g. Alq3, or any other small sublimed molecule or conjugated polymer electroluminescent material as known in the prior art. The material may be deposited by vacuum sublimation.
According to a second aspect of the present invention there is provided a method for forming an organic light-emitting device, comprising the steps of: depositing a region of a non-light-transmissive layer over a transparent cover sheet; depositing a region of organic light-emitting material over an area of the transparent cover sheet that is not covered by the non-light-transmissive layer; depositing over the non-light-transmissive layer a region of circuitry for regulating the flow of current to the organic light-emitting material.
The non-light-transmissive layer may be deposited by sputtering or vapour deposition. Other preferred features of the method correspond to those described above in relation to the first aspect of the invention.